1. Field of the Invention
The present invention relates to a charged particle radiation apparatus that controls the beam convergence characteristics of charged particle radiation. More specifically, the present invention relates, for instance, to a focused ion beam apparatus that is capable of producing a thin beam even when a large beam current flows at various acceleration voltages.
2. Description of the Related Art
In recent years, a technology for microfabricating a sample by irradiating it with a thin, high-energy charged particle beam, particularly, with an ion beam, has been frequently used. When a sample is to be microfabricated with an ion beam, the speed and sophisticatedness of such microfabrication increases with an increase in the current of the ion beam and with a decrease in the diameter of the ion beam. It is therefore demanded that a focused ion beam apparatus control the beam convergence characteristics of an ion beam.
In a book titled “Electron/Ion Beam Optics,” which is written by Katsumi Ura and published in 1994 by Kyoritsu Shuppan, the basic configuration of a focused ion beam apparatus is described. When controlling the beam convergence characteristic of an ion beam, a focused ion beam apparatus uses an electrostatic lens, which functions in accordance with an electric field distribution, instead of an electromagnetic lens, which functions in accordance with velocity and magnetic field distribution, because ions are heavy in mass and lower in velocity than electron beams by approximately two orders of magnitude.
The electrostatic lens (objective lens) is positioned opposite a radiation sample to radiate a thin beam onto the sample. The electrostatic lens is generally composed of three electrodes. The ion beam can be reduced to a small diameter by applying a positive or negative voltage to the intermediate electrode. The electrostatic lens exhibits more significant chromatic aberration than the electromagnetic lens. Therefore, the limit imposed on ion beam diameter reduction by the electrostatic lens generally depends on the chromatic aberration of the electrostatic lens.